I strapped 2 5000mAh batteries to my F450 and did one test flight. I got 20 min flight time which I was quite happy with.

My next flight was going to be another long distance test up the Penarth coast. Flatholm is an island about 4km of the coast, which has been a tempting target for some time, so I thought with the extra battery capacity, I'd see how far I could get.
I covered 3.5km in about 4min, and then started to lose video, so I started back in what should have been plenty of time. The wind was against me on the return leg, but I was still making decent headway, but the battery voltage was dropping like a rock.
I was now panicking because I could see I was not going to make it back. I tried everything, but in the end it crashed into the sea less than 100m from dry land. In the end the batteries only lasted 10min...so I guess I should have done more testing, and just stuck to the costal flights.